[lifthrasiir]

Using TAI exclusively is simpler, but that's not the parent advocates (and neither do you, I believe).

Using TAI primarily and UTC in display has two main benefits that I consider marginal. The first is that the addition and subtraction is associative and transitive, but their use towards non-zoned wall clock timestamps like time_t is limited anyway; they normally operate on zoned date & time (e.g. "tomorrow" typically refers to the local time) or on monotonic and steady timestamps with no particular epoch. The second is that the timestamp doesn't repeat but this can also happen with TAI, just that it can only happen with (a large amount of) clock synchronization; you should be prepared of wall clock timestamps repeating no matter it's UTC or TAI.

For those reasons I consider that using TAI primarily and UTC in display is not much superior to using UTC primarily. Not that I don't wish TAI to be universal, but given the current circumstances the switch doesn't justify its cost.